Method of electrostatic duplicating by image transfer

ABSTRACT

An organic photoconductive member is formed by applying an organic photoconductive film over a polyester-conductive substrate such as aluminized &#39;&#39;&#39;&#39;MYLAR&#39;&#39;&#39;&#39; and includes as a separate layer a resistive current limiting layer such as polyvinyl alcohol having a thickness of 3 to 15 microns so that the photoconductive member may be successfully employed in the environment of a copying machine in which an electrode is employed at the developing station, said resistive layer serving to prevent electrical shorting between the electrode and the conductive layer that may occur through any holes in the photoconductive film thereby preventing the uncontrolled deposition of toner across the contact width of the developer where the hole occurs.

United States Patent [191 Gasner Nov. 18, 1975 [75] Inventor: Earl L. Gasner, Arlington Heights, 111.

[73] Assignee: Addressograph-Multigraph Corporation, Cleveland, Ohio [22] Filed: May 6, 1974 [21] Appl. No.: 466,935

Related U.S. Application Data [63] Continuation of Ser. No. 221,500, Jan. 28, 1972,

abandoned.

[52] U.S. Cl 96/L4; 96/1 R; 96/1 SD; 96/l.5; 355/3 R; 355/3 DD [51] Int. Cl. G03G 13/22 [5.8] Field of Search 96/1 R, 1 SD, 1.4, 1.5

[56] References Cited UNITED STATES PATENTS 2,860,048 11/1958 Deubner 96/1.5

2,901,348 8/1959 Dessauer 96/1.5

3,158,475 11/1964 Cassiers et a1. 96/1.5

3,170,790 2/1965 Clark 96/1.5

3,262,806 7/1966 Gourge 96/1 R X 3,393,070 7/1968 Snelling 96/l.5 3,438,773 4/1969 Hayashi et a1. 96/1 R 3,589,895 6/1971 Ville 96/1.4 3,598,580 8/1971 Baltazzi 96/l.4 3,713,821 l/l973 Angelini 96/l.5 3,723,111 3/1973 Kojima et a1. 96/1.5 X

Primary E.raminerRoland E. Martin, Jr.

[5 7] ABSTRACT An organic photoconductive member is formed by applying an organic photoconductive film over a polyester-conductive substrate such as aluminized MY- LAR and includes as a separate layer a resistive current limiting layer such as polyvinyl alcohol having a thickness of 3 to 15 microns so that the photoconductive member may be successfully employed in the environment of a copying machine in which an electrode is employed at the developing station, said resistive layer serving to prevent electrical shorting between the electrode and the conductive layer that may occur through any holes in the photoconductive film thereby preventing the uncontrolled deposition of toner across the contact width of the developer where the hole occurs.

8 Claims, 5 Drawing Figures U.S. Patent Nov. 18, 1975 Sheet10f2 3,920,453

jmin

v ofgyzsner METHOD OF ELECTROSTATIC DUPLICATING BY IMAGE TRANSFER This is a continuation of application Ser. No. 221,500 filed Jan. 28, 1972, now abandoned;

BACKGROUND OF THE INVENTION This invention relates generally to an improved process of photoelectrostatic duplication and more particularly relates to a duplicating process in which the photoconductive member is reused in an environment where one or more high voltage electrodes and/0r biasing electrodes are used in the reproduction process.

Photoelectrostatic duplicating processes, in which a photoconductive member is imparted a powder image which is then transferred to a sheet of plain paper, are generally well known. These duplicating processes require that the photoconductive layer be applied a blanket sensitizing charge, followed by exposure to a pattern of light and shadow in order to createa charge image on the photoconductive surface. The charge image is visualized by development with an electroscopic powder. In many instances the developing device is equipped with a biasing electrode which applies a potential gradient between the conductive backing of the photoconductive member and the electroscopic powder. The need for such biasing electrodes depends on the particular system such as described, for example, in U.S. Pat. No. 3,598,580, granted on Aug. 10, 1970, to Evan S. Baltazzi, et al., entitled Photoelectrostatic Copying Process Employing Organic Photoconductors and assigned to the same assignee, in which the photoconductive layer is underexposed and through the use of a biasing electrode at the developing station, the electrical field in the background area is rendered ineffective to attract toner.

The final step in the duplicating process requires the transfer of the powder image that has been developed at the developing station to-a receiving sheet. Again,

. the transfer station utilizes electrodes to facilitate the movement of the powder particles from the photoconductive layer to the receiving sheet. Such electrodes may be corona discharge electrodes which impart a voltage to the receiving sheet having a polarity opposite to that of the toner particles causing the particles to move in the direction of the receiving sheet. A noncorona electrode may be employed which produces a field effect, such as a transfer roller having a potential applied thereto causing the powder to move in the direction of the receiving sheet.

It will be appreciated that the duplicating techniques referred to above, require the use of electrodes conin this description be in the form of a continuous belt.

It may also be in the form of a web in which case the photoconductive member is carried on spools, being unwound from one spool and taken up on a wind-up spool. The spools are mounted in a rotatable drum in a manner which permits a portion of the photoconductive member to be disposed on the drum surface. Rotating the drum cycles the photoconductive member past the various image producing processing stations.

The construction of a typical photoconductive member provides a base support formed of a polyester film to which is applied a conductive layer such as, for example, aluminum. Aluminized mylar serves as an excellent support over which is applied a film of photoconductive material. A complete circuit is required from the corona wire through ground in order to charge the film; during development, a conductive path is required during the application of toner so that it will properly be deposited on the charge pattern; and during transfer, a complete circuit is required from the transfer electrode through ground to cause the powder to transfer to the receiving sheet.

Such a photoconductive member in the environment of an apparatus equipped with electrodes as described hereinabove, requires that the photoconductive film layer be uniform and continuous, free of any apertures or pin holes which would provide a direct connection between the conductive layer and the high voltage source. In the circumstance that there is a weakness, irregularity or failure in the photoconductive layer, there will result a short circuit between the electrode at the developing station, which will allow deposition of toner onto the photoconductive layer coextensive with the dimensions of the developer applicator device. Such uncontrolled deposition of toner to the photoconductive sheet adjacent the areas where the short circuit occurs obliterates the image portions and hence, the member is no longer usable.

SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide a duplicating process for making reproductions on plain paper in which the construction of the photoconductive member permits a greater latitude in the range of operating parameters.

It is a specific object of the present invention to provide a duplicating process for making reproductions on plain paper in which consistently high quality copies can be obtained using high voltage biasing techniques without the risk of reducing the useful life of the photoconductive member.

It is a more specific object of the present invention to provide an improved copying process in which there are employed high voltage biasing electrodes at the various processing stations and which utilize a photoconductive member having a protective resistive layer so that the reproduction process is protected against the deleterious effects on the photoconductive member.

It is a further specific object of the present invention to provide a process which uses a photoconductive member constructed with a resistive layer in contact with the photoconductive layer to prevent the formation of short circuits through the photoconductive layer at the processing stations which require high voltage biasing electrodes.

It is still another further specific object of this invention to provide an improved photoconductive member which is comprised of a conductive base support, a photoconductive layer and a resistive layer in contact with the photoconductive layer to prevent short circuiting currents to flow through irregularities in the photoconductive layer.

Briefly, in the preferred embodiment of the instant invention, the process of making a reproduction from a photoconductive medium comprises the steps of applying a blanket electrostatic charge to a specially constructed photoconductive medium by means of a conventional corona charging electrode; a charged photoconductive member is then exposed to a pattern of light and shadow resulting in the creation on the photoconductive member of a charge pattern. The charge pattern on the photoconductive member is developed by applying an electroscopic powder thereon utilizing a developing device, such as a magnetic brush, while imposing a potential gradient between the conductive base'support'of the electrophotographic member and the magneticbrush. The material image which is developed on the photoconductive layer is then transferred to a receiving sheet, such as plain paper under the influence of a transfer electrode, which applies a field between the surface of a powder image on the photoconductor and the plain paper which the image is to be transferred.

The success of the photoconductive member in withstanding exposure to the various high voltage electrodes as it courses past the processing stations, has been achieved by the novel and special construction of the photoconductive member thereby obviating the need for providing a critically uniform, uninterrupted photoconductive layer. The special construction involves contacting the photoconductive layer with a protective resistive layer so as to provide an impedance between the developing brush and ground or the reference potential. The resistivity of the photoconductor in its dark adapted condition is about 10 to 16 ohm centimeters; the resistivity of the protective resistive.

layer should be in the range of to 10 ohm centimeters.

The value of the resistivity for the current-limiting layer has been determined to be a function of the rate of movement of the photoconductor past the developing brush or electrode, and the biasing potentials applied. In general it can be stated that the layer must have a time constant which prevents loss of bias voltage at the site where there is a break in the photoconductive layer.

The general requirements of such a protective resistive layer is that it have an electrical resistance high enough to prevent the formation of shorting currents at the level where they do not result in loss of bias voltage at the photoconductive surface and thereby prevent the localized attraction of toner. The protective layer must also have a dielectric strength that is sufficiently high to prevent voltage breakdown. The resistivity should be low enough in order to produce negligible currentlimiting effects on the normal charging, developing and transferring operations under the influence of high voltage electrodes. It is to be understood that the protective layer must intervene between the electrode and the conductive layer and hence, it may be applied over the photoconductive layer or between the conductive layer and the photoconductive layer.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a side elevation of a duplicating apparatus capable of carrying out the process of the instant invention.

FIG. 2 is a schematic wiring diagram showing the various electrodes disposed about the photoconductive member.

FIG. 3 is a schematic in cross section showing one embodiment of a photoconductive member useful in the instant invention.

FIG. 4 is a schematic in cross section of another embodiment of the instant invention.

FIG. 5 is a schematic illustrating the improved performance characteristics of the photoconductive members of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Having reference now to the drawings, and particularly FIG. 1, there is illustrated a copy machine 10 which is adapted to carry out the process of the instant invention. The copy machine 10 is of the photoelectrostatic type which is enclosed in a cabinet or housing 12, and is equipped with an infeed station 14, into which are fed the originals to be reproduced. At the lower portion of the housing there is provided a stack supply of cut sheets 16 from which supply are fed the copy sheets 17 or receiving members to which are transferred the powder images. The copy sheets 17 pass through a fixing station 18 and are delivered into a copy receiving tray 20 at the front of the housing 12.

In the forward portion of the housing is a photoconductive medium identified generally as 22, which is in the form of a continuous belt. The construction of the photoconductive medium 22 is critical to the successful operation of the instant process, as will be described in greater detail hereinafter. The photoconductive medium comprises a polyester base support 24 over which is applied a conductive layer 26. Directly in contact with the conductive layer 26 is the resistive protective layer 30 over which is next applied the photoconductive layer 32. It will be appreciated that the resistive layer 30 may be applied over the photoconductive layer 32. The continuous belt 22 is stretched about a plurality of rotatable rolls 34, 36, 38 and 40, which are mounted for rotation about horizontal parallel axes a, b, c and d, respectively. One of the rolls such as 40, is swingably mounted on two bracket members 42 and 43 and is designed for yieldably biased displacement away from the belt path to serve as a belt tensioning control. Another of these rolls, for example 36, is caused to be positively rotated through shaft b by a suitable driving mechanism comprising a motor 44 which drives the sprockets 41a and 41b through a drive belt 46, contained within the housing 12.

Surmounting the photoconductive medium 22 is a swingably mounted superstructure 47 formed of a pair of L-shaped members 48 and 49, only one such L position overlying the flat portion 58 of the path taken by the belt 22 passing over the rolls 34 and 38.

On the underside of the superstructure 47 is a corona discharge device 60 which includes a fine wire electrode 62 stretched inside a conductive shield 64. The corona discharge device extends between the members 48 and 49 being affixed to the tie bar 56. The corona discharge device is mounted in spaced relation above the photoconductive member 22, three-eights distance of from one-half to three-eighths of an inch, as the member passes over the roll 38. The photoconductive member 22 is imparted a blanket electrostatic charge at this station.

Included on the underside of the superstructure at the portion spanning the rolls 34 and 38, are a pair ol rotatable rolls 66 and 68 mounted for rotation about the shafts e and f, which. as shown in the drawings, are

parallel to the shafts a and c, and which reside inside the lateral dimension between the rolls 34 and 38 extending transverse the full width of the member 22. The belt, as it courses between the two sets of rollers 34 and 38 and 66 and 68, is supported from below as well as above, to provide a taught, flat, optical exposure plane 58.

An elongated radiation source 72 is supported in a suitable reflector housing 74 which straddles the belt and extends the width thereof providing a source of radiant energy for the photoconductive member 22. The reflector housing is mounted in the cut-out portion 76 of the bracket 48. The roll arrangement also serves to advance the original, which is to be reproduced, into intimate contiguous contact with the photoconductive medium 22 over the entire exposure plane 58.

In operative association with the roll 34 is the developer assembly 80 for applying the electroscopic powder to the latent image bearing photoconductive layer 32 of the photoconductive member 22 as it leaves the exposure station 58. The developer assembly includes a trough 82 in which is contained a supply of developer powder 84 which can be a conventional mixture consisting of iron carrier particles with electroscopic thermoplastic powder of the type used in magnetic brush developers. The mix 84 is applied by the applicator 85. The construction and operation of magnetic brush developers is well known and will not be discussed in detail here. It has been found necessary in the mounting of the developer asembly described hereinbefore that it be electrically floating, that is, insulated from the rest of the apparatus. The trough is supported on a shelf or platform 86 comprised of a highly insulating plastic material, such as acrylic or polystyrene resins, or the equivalent, which in turn is supported by the frame structure of the machine. A drive motor 85 is provided for driving the magnetic brush assembly 80. The electrical circuitry and controls for the machine of FIG. 1 will be discussed in greater detail hereinafter. It has been found desirable to apply a negative potential to the developer in the range that is equal to or slightly greater than the voltage in the background area. A more detailed description of this developing and exposure technique may be found in US. Pat. No. 3,598,580 granted Aug. 10, 1970, to Evan S. Baltazzi, et. al., and assigned to the same assignee as the instant application.

It will be appreciated that the application of the developer powder has been described in terms of a magnetic brush assembly; however, it will be understood that other types of powder development may be used, such as, for example, a cascade development system which employs conductive glass beads and which may have a development electrode juxtaposed the area where the powder cascades onto the photoconductive member.

Transfer of the material image to the copy sheet is accomplished in the transfer station 88 to which a copy sheet 17 is fed from the stack 16 by means of the feed wheel 90 in timed relation to the arrival of the image carried on the surface of the belt at the transfer station. In pressure contact with the roller 36 is an adjustable pressure assembly 92 which includes a transfer roll 94 journaled for rotation in an insulating bearing 96 which in turn is received in pivotable yoke member 98 which is pivotally secured a frame 99 at the point 100.

One end of the yoke member 98 is. attached to a threaded post unit 101 which is secured at one end to the housing 12 and the other end being rotatably received in the yoke. At the base of the post is a threaded nut 102 and surrounding the post 101 captured in a compressed condition between the yoke 98 and the threaded nut 102 is a coiled spring member 108. Adjustment of the nut 102 increases or decreases the amount of compression on the spring and hence the corresponding force with which the yoke and roller 94 are urged against the roller 36. i

It is desirable during the transfer mode to adjust the pressure in the range from about 2 pounds to about I00 pounds per lineal contact inch.

Referring to FIG. 2 of the drawings, there is shown a schematic representation of the various electrodes disposed about the belt member 22 as it rotates about the drive rollers 34, 36 and 38 in a predetermined path past the charging assembly 60, the developing assembly and the powder image transfer station 92. The electrical supply of the apparatus of FIG. 1 of this invention is represented in the wiring diagram of FIG. 2 showing feed lines and 112 connected across a 1 15 volt AC supply which feeds the high voltage powder circuit.

The high voltage power supply circuit includes separate power supplies 114, 116 and 118 which are connected across the feeder lines. The supply 114 provides a voltage to the charging device 60, supply 116 provides a voltage to the transfer roll 94 and the supply 118 provides a variable voltage to the developer assembly 80.

The corona wire 62 is connected via connector 120 to the negative terminal of the high voltage supply 1 14 applying a voltage in the range of 4000 to 6000 volts to the fine wire 62 producing a corona emission there from. As the photoconductive member 22 passes beneath the wire 62 at the charging station 60, a blanket electrostatic charge is applied to the surface thereof.

As the belt 22 continues to move out from the charging station 60, a graphic original is fed into the system via the infeed station 14 where it is brought into contiguous and intimate contact with the charged surface of the member 22 via the rollers 66 and 68 as shown and described in connection with FIG. 1. At the exposure station 58, a pattern of light and shadow is imparted to the photoconductive member by the electromagnetic radiation source 72.

Leaving the exposure station, the belt moves to the next station which is the developing station 80 at which toner is applied to the latent image bearing photoconductive surface. As has been explained earlier, it has been found desirable that the development process be carried on in the presence of a field between the developer particles and the charge image. This requires that the developer apparatus be connected to the negative terminal via the connector 122 to the negative terminal of the high voltage variable supply 118. It has been found desirable to apply a negative potential to the developer in a range that is equal to or slightly greater than the voltage in the background area of the photoconductive member 22. The bias potential applied to brush is in the range of to 300 volts.

Transfer of the powder image which is applied by the developer assembly 80 is transferred to the copy sheet 17 which is fed from the supply 16 by means of the feeder wheel 90 (FIG. 1) in timed relation to the arrival of the powder image at the transfer station. In pressure contact with the roll 36 is a transfer roll 94 which causes the powder image to transfer to the copy sheet 17. The transfer roll 94 operating in conjunction with 7 the roll 36 will cause the powder to transfer to the copy sheet which is passed between these rolls.

In the circumstance that the apparatus is to be operated in a duplicating mode, that is, where the powder image is transferred from the surface of the photoconductive member 22 and thereafter the latent image is successively redusted and transferred, ,it becomes necessary to apply a field between the surface of the powder image and the plain paper that is fed from the stack 16. To create the proper environment for the duplicating mode, a DC voltage is applied to the transfer zone between the two rollers, 36 and 94, by connecting the metal shaft 124 of the roller 94 to the high voltage DC supply 116 applying a field having a polarity which is the same as the polarity of the electrostatic charges in the latent image portions. Accordingly, the core 124 is connected to the negative terminal of the source 116 via the connector 126. The voltage applied to the conductive core of the roll-124 is in the range of from 1000 to 3000 volts, preferably in the range of from l200 to 2000 volts, and the pressure in the range of from 2 to 8 pounds per square inch contact area.

Referring to FIG. 3, there is shown in schematic cross-section the construction of the photoconductive member 22 that is responsible for the successful operation of the process which is carried out by the apparatus 10.

In order to provide a photoconductive member that is sufficiently flexible and strong so that it may be used in the form of a continuous belt with the apparatus 10, the base 24 is preferably formed of a polyester film. The photoconductive member 22 may be utilized in other forms such as a web which is wound or uwound from spools on suitable mountings and dispensed in appropriate lengths to a suitable supporting means for sequential processing.

The photoconductive member 22 is formed of a flexible base support material 24 such as a polyester or other plastic substances such as cellulose acetate, cellulose triacetate, cellulose acetate butyrate, polyurethane elastomers, fluoro carbon films such as the copolymer of hexafluoropropylene, and tetrafluoroethylene, cellulose propionate, ethyl cellulose, polyamide, polymethylmethacrylate, polyethylene, ethylene vinyl acetate copolymer, polyvinylfluoride, polypropylene, polyester terephthalate, polytetrafluoroethylene, polystyrene, polyvinyl alcohol, polyvinyl chloride, vinylidene chloride-vinyl chloride copolymer, polycarbonate and rubber hydrochloride. The preferred film material is polyester terephthalate film sold under the trademark MYLAR which offers the greatest dimensional stabil- The resistivity value of the polyester-type films is high, being in the range of to 10 ohm centimeters. These polyester materials, because of their high I resistivity, are unsuitable as base supports on which the photoconductive material may be directly applied since it is important that the base support have a resistivity in the range of from l0 ohm centimeters to the resistivity of metal.

The conductivity of the layer 36 is achievedby applying a thin, uniform metallized layer such as aluminum or copper by vapor deposition which may range in thickness from 0.005 mil to 0.50 mil. Other conductive treatments may be applied which involve a metal-containing semi-conductive compound that is dispersed in a film forming binder and a semi-conductor and binder are then both dissolved to form a coating solution.

Since most semi-conductors are not readily soluble in organic--solvents, ;they arecombined with other compounds to form solubilizing complexes. The semi-conductive mater-ialsthat are usable are cupric and silver halides,- halides1of-bismuth, gold, indium, iridium, lead, nickel palladium, 'rhenium, tin, tellurium and tungsten. The semi-conductors are solubilized by complexing and then-being dispersed in a-binder such as polyvinyl acetate and the mixture dissolved in cyclohexanone or other more suitable ketones.

In direct contact with the conductive layer is next applied theprotective resistive layer 30. The general requirement of such a resistive layer is that it must have an electrical resistance which is sufficiently high to prevent shorting currents from occurring due to defects or irregularities in the photoconductive layer; it must have a dielectric strength sufficiently high in order not to break down upon being subjected to high fields. The resistance of the resistive layer should be sufficiently low in order to produce negligible effects on the normal charging, exposing and developing operations neces sary to produce an image. It has been found that a suitable protective layer may be formed by using one of the following film forming materials.

methyl cellulose hydroxypropyl methyl cellulose ethylacrylate-acrylic acid-styrene terpolymer copolymer of methyl vinyl ether and maleic anhydride polyvinyl alcohol The above materials are dissolved in a suitable solvent system, in some instances the solvent may be water and where necessary, a curing agent is to be used.

The resistive protective layer is applied at a rate sufficient to yield a dry film thickness in the range of 3 to 15 microns, preferably in the range of 6 to 10 microns. The operable range of thickness is from 2 to 50 microns.

Following the application of the resistive protective layer from which the excess solvents had been evaporated and the film is brought to a dry condition, and there is applied the photoconductive layer 32 which may be a photoconductive pigment, in a resin binder such as zinc oxide, cadmium, sulfide, cadmium selenide. To equal advantage, the photoconductive layer may be an organic photoconductive system selected from a wide range of aromatic hydrocarbons such as disclosed in US. Pat. No. 3,287,199, to I-Ielmut I-Ioegl, et. 211., issued Nov. 22, 1966. Such photoconductors include aromatic hydrocarbons such as naphthalene, anthracene, benzanthrene, chrysene, peradiphenylbenzene, diphenylanthracene, pera-terphenyl and peraquaterphenyl, and sexiphenyl; heterocyclic compounds such as oxidiazoles; triazoles; imidazolones and imidazothiones; N-aryl pyrazolones; hydrated imidazoles; and other compounds. The organic photoconductive substances may be applied to the base support from a solvent solution or in conjunction with a film forming binder such as a natural resin of which shellac is an example; a synthetic'resin such as a coumarone resin, or such binders as cellulose ethers, vinyl polymers, polyacrylates.

Another group of photoconductive materials which work eminently well are organic polymeric photoconductors. The classes of polymeric substances that have been found to be-useful in the practice of the process of the instant invention are vinyl polymers, and more particularly vinyl carbazole and vinyl copolymers contain- 9 ing vinyl carbazole units. In the circumstance where the photoconductor is an organic polymeric material, it will serve as its own film forming agent and will not require the use of a separate binder. Such photoconductive materials as polyvinylcarbazole and poly(N-vinylbenzocarbazole) give excellent results.

Referring to FIG. 4, there is shown another embodiment of this invention identified generally as the photoconductive member 130. The base support is similar to that which is employed in the photoconductive member of FIG. 3 and it comprises a base support 134 over which is applied a conductive metal layer 136 and the photoconductive layer or film 138. Surrnounting the photoconductive layer is the protective resistive layer 140. It will be appreciated that the difference between the photoconductive member 130 and 22 is that the former has the protective resistive layer residing on top of the photoconductive layer. With the exception of the location of the protective resistive layer 140 of the photoconductive member-130, it is similar in construction in all other respects to the photoconductive member 22.

The following examples will describe the preparation of the special photoconductive materials which give rise to the novel duplicating process of the instant invention.

EXAMPLE I To a base support consisting of an aluminized mylar in which the mylar film thickness was 3 mls and the aluminized conductive layer was 0.002 mils, there was applied a resistive protective layer having the following formulation:

polyvinyl alcohol 1 part water 9 parts polymeric quaternary amine salt 0.5 part identified as:

ECR-34 sold by Dow Chemical Company sufficient to produce a photoconductive layer of about 0.2 to 0.3 mils in thickness after the evaporation of the solvent.

The finished photoconductive member was formed into a continuous loop by a suitable welding technique for welding the mylar and disposed about the rollers 34, 36, 38 and 40. It should be noted that the construction of the belt employs the invention described in US. Pat.

No. 3,533,692, issued on Oct. 13, 1970, to Robert G. I

Blanchette et al and assigned to the same assignee as the instant invention. In the subject US. patent, there is disclosed a technique for continually grounding the conductive layer 26 to the transport rollers through the use of conductive connections which penetrate the 10 mylar layer providing a conductive connection between said layer and the roller.

EXAMPLE II The photoconductive member of this example was prepared in the same manner as described in the preparation of Example I with the exception that the protective resistive layer was formulated as follows;

Methyl cellulose 0.20 parts identified as: METHOCEL MC sold by Dow Chemical Corporation Quaternary ammonium salt 0.06 parts identified as: ARQUAD C-SO sold by Armour Industrial Chemical Water l0 parts This resistive protective coating was applied at a rate sufficient to provide a dry film thickness of 6 microns.

EXAMPLE III The photoconductive member of the instant example was prepared in the same manner as Example I with the exception that a protective resistive layer having the following formulation was substituted for the resistive layer of Example I.

Hydroxypropylmethyl cellulose 2.4

identified as: METHOCEL 65HG sold by Dow Chemical Company U FORMITE 700 1.2 Rohm and Haas Company Organic Quaternary amine salt identified as: ARQUAD C-50 sold by Armour Industrial Chemical Methanol 8 Water 7 Hydrochloric acid (sufficient to give pH of 4.5)

parts parts parts parts parts The solution was applied at a rate sufficient to produce a dry film coating thickness of 5 microns. Prior to the application of the photoconductive coating, it was necessary that the film be permitted to cure over night in an oven at 100 C.

EXAMPLE IV The photoconductive member of this example was prepared in the same manner as the photoconductive member of Example I with the exception that a protective resistive layer having the following formula was substituted for the resistive layer described in Example I.

Diisocyanate resin identified as:

DDl sold by General Mills Company Modified amine identified as:

A-lOO sold by General Mills Company Organic'quaternary amine salt identified as:

ARQUAD 2HT sold by Armour lndustrial Chemical Toluene parts 4.8 parts parts 5 8 parts EXAMPLE V The photoconductive member of this example was prepared following the procedures set forth in the description of the Example I with the exception that the protective resistive layer was substituted with a layer having the following formulation.

Ethyl acrylate/acrylic acid/ styrene terpolymer in a ratio of 69, 8 and 23 weight ratio Epoxy resin identified as: EPON 836 sold by Shell Chemical Corporation Organic quaternary amine salt identified as: v ARQUAD 2HT 75 sold by Armour Industrial Chemical n-butanol Toluene 2. l parts 0.5 parts 05 parts parts parts LAN The resistive coating of this example required curing at 100 C for 24 hours. It was applied at a rate sufficient to result in a dry coating thickness of 5 mils.

EXAMPLE VI The photoconductive member of this example was prepared following the general procedures set forth in Example I with the exception that a protective resistive layer having the following formulation was substituted for the formulation shown in Example I.

Copolymer of methyl vinyl ether and maleic anhydride parts identified as: GANTRY AN 119 sold by General Analine and Film 1,5-pentanediol 1.27 parts Dibutylphth alate 1.0 parts Organic quaternary amine salt 4.9 parts identified as: ARQUAD C-SO sold by Armour Chemical Company 90 parts Tetrahydrofuran The resistive coating formulation was applied at a rate sufficient to produce a dry film thickness of 6 microns and required curing for a period of 24 hours at 100 C.

EXAMPLE VII Resistivity Of Dark Resistivity The Protective Layer Photoconductive l2 7l20% Relative Ex. Layer Relative Humidity Humidity 1 l0"l0"" ohm cm. 2X10" ohm cm. less than I0 ohm cm. 2 less than l0 ohm cm.

3 H H 1, 4 6X10 ohm cm. 5 8X10 ohm cm. 6 2X10 ohm cm.

It will be appreciated that the resistive is humidity sensitive and its resistance will be a function of humidity as shown in the preceding chart. In each of the examples the resistive layer performed satisfactorily over the humidity ranges shown.

In utilizing the photoconductive members on the apparatus which is described in connection with FIG. 1, the photoconductive layer was charged to a level of about 800 volts via a corona charging unit 60.

At the exposure station, the photoconductive member was exposed to a pattern of light and shadow for a period of time sufficient to reduce the voltage in the light struck areas to a level of about 300 volts. The photoconductive member with its pattern of light and shadow was advanced to the developer brush where it was applied the electroscopic powder. The magnetic brush assembly was connected to the negative terminal of a DC voltage supply and the other terminal of the supply was connected to ground in order to apply a negative potential to the developer in the range that was equal to or slightly greater than the voltage in the background area, which in the instant example was about 300 volts.

It will be appreciated that the brush was electrically biased with a potential from a source having a polarity which was the same as the polarity of the sensitizing charge. The toner powder in this environment was selectively attracted to the image areas to the exclusion of the background areas, which was at a rather significant level of 300 volts. The presence of the protective resistive layer 30 prevented any short circuit of the voltage impressed on the magnetic brush to ground through any irregularities or apertures in the photoconductive layer 32.

Referring to FIG. 5, there is shown the operation of the improved photoconductive member 22 of this in- .vention under the conditions wherein there is a break or discontinuity in the photoconductive layer 32. It will be seen that the resistive layer 30 acts as a resistance 140 of FIG. 5 connected between the photoconductive layer 32 and the vaporized metal layer 26.

As shown in FIG. 5, the member 22 is shown being passed beneath the developer brush on which is carried the developer mix 84. The portions of the photoconductive surface which were not light exposed, and which retain the charge applied by the corona assembly 60, attracted the toner portions 84' from the developer mix 84 producing a visible image on the surface. As the brush passes over the opening 142 in the photoconductive layer 32, which opening may be a pinhole or such other irregularity as to expose the surfaces immediately beneath the opening 42 directly to the high voltage source 118. The presence of the layer 30, actingas if it were the resistance I40, prevents the flow of current 13 which would otherwise resultbya direct connection of the high voltage source 118 tothe conductivelayer 26 and thence to ground. Such a direct connection would cause loss in bias voltage and the deposition of-the particles 84' to be deposited transverse the surface 32 corresponding dimensionally to the length of the applicator roller 85. The blackened area caused by such an uncontrolled deposition of toner in the brush contact area could be as much asone-half inch in width extending the transverse dimension of the photoconductive member. The attraction and deposition of toner across the entire width of the photoconductive member in the manner described is identified as a shorting bar. The formation of such a shorting bar is completely avoided by the use of a photoconductive member having the special construction shown in FIG. notwithstanding the fact that the photoconductive layer 32 has the opening or pinhole 142 in its surface. Preventing against the formation of a shorting bar protects the receiving sheet from the attraction of a band of toner.

The process of the instant invention has been disclosed in some detail and it is intended that such detailed description is for the purpose of providing a complete teaching and is not intended to limit the scope of the invention which is defined in the appended claims.

What is claimed is: l. The process of making reproductions from a reusable photoconductive medium comprising the steps of:

a. forming a photoconductive medium consisting essentially of a conductive base support carrying thereon an organic photoconductive layer and a resistive current limiting layer containing a filmforming material selected from the group consisting of methyl cellulose, polyurea, hydroxypropyl methyl cellulose, ethylacrylate-acrylic acid-styrene terpolymer, copolymer of methyl vinyl ether and maleic anhydride and polyvinyl alcohol, said current-limiting layer having a thickness of from about 2 to about 50 microns in contact with said photoconductive layer, the resistivity of said resistive layer being in the range of to 10 ohm-centimeters; 1

b. applying a blanket electrostatic charge to the photoconductive medium by means of a charging electrode;

c. exposing the charged layer to a pattern of light and shadow to produce a charge image thereon;

(1. developing the charge image by applying an electroscopic powder therein while imposing a potential gradient between said conductive base and the electroscopic powder by means of a biasing potential;

e. transferring the powder image to a receiving member under the influence of a transfer electrode;

f. establishing a reference potential for the electrodes in steps (b), (d), and (e), by providing a reference electrode;

g. repeating at least once the steps (b) through (f);

whereby loss in the biasing potential at said developing step is prevented by said resistive layer interrupting the direct connection of said photoconductive layer to said reference electrode in the event of any discontinuity in said photoconductive layer.

2. The process of successively developing and transferring a powder image from an electrophotographic member bearing a latent electrostatic image with an electroscopic powder by means of a magnetic brush developer comprising the steps of:

a. forming a photoconductive medium comprising a conductive. base support carrying thereon an organic photoconductive layer and a resistive layer containing a film forming material selected from the group c'onsisting'of methyl cellulose, polyurea, hydroxypropyl methyl cellulose, ethylacrylateacrylic acid-styrene terpolymer, copolymer of methyl vinyl ether and maleic anhydride and polyvinyl alcohol, said resistive layer having a resistivity in the range of 10 to 10 ohm-centimeters and having a thickness in the range of 2 to 50 microns in contact with said photoconductive layer;

b. connecting said conductive base support to a reference electrode;

. applying a biasing potential to said magnetic brush thereby establishing a potential gradient between said conductive base and said electroscopic powder; whereby any loss in said biasing potential is prevented by said resistive layer interrupting the direct connection of said photoconductive layer to said reference electrode in the event of any discontinuities in said photoconductive layer;

d. and thereafter developing the charge image by applying said electroscopic powder thereon;

e. and transferring the powder image to a receiving member under the influence of a transfer electrode; and

f. repeating at least once the steps (c) through (e).

3. The method as claimed in claim 2 wherein the bias potential applied to the magnetic brush is in the range of 50 to 300 volts.

4. The method as claimed in claim 2 wherein the reference electrode is at ground potential.

5. The process of latent image duplication for making copies of a graphic original using a photoconductive medium comprising the steps of:

a. forming a photoconductive medium comprising a conductive base support carrying thereon an organic photoconductive layer and a resistive current-limiting layer with a thickness of from about 2 to about 50 microns in contact with said photoconductive layer, the resistivity of said resistive layer being in the range of 10 to 10 ohm-centimeters, said resistive current-limiting layer further characterized as containing a material selected from the group consisting of methyl cellulose, hydroxypropyl methyl cellulose, ethylacrylate-acrylic acid-styrene terpolymer, copolymer of methyl vinyl ether, maleic anhydride, polyvinyl alcohol and polyurea;

b. applying a blanket electrostatic charge to the photoconductive medium by means of a charging electrode;

c. exposing the charged layer to a pattern of light and shadow to produce a charge image thereon;

d. developing the charge image by applying an electroscopic powder by means of a magnetic brush while imposing a potential gradient between said conductive base and the electroscopic powder by connecting said magnetic brush to a power source imposing a biasing potential at said developer;

e. transferring the powder image to a receiving member under the influence of a transfer electrode connected to a voltage source establishing a potential gradient between the transfer member and said conductive support imposing a biasing potential during transfer;

.establishing a reference potential for the electrodes in steps (b), (d) and (e), by providing a reference 6. The method as claimed in claim wherein the biasing potential at said developing station is in the range of 50 to 300 volts.

7. The method as claimed in claim 5 wherein the biasing potential at the transfer station is in the range of 1,000 to 3,000 volts.

8. The method as claimed in claim 5 wherein the polarity of the applied voltage is the same as the polarity of said blanket electrostatic charge.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5,92OJP55 Dated November 18, 1975 Inventor s) Earl L llel" It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Columns 1 through as shown on the attached sheet should be added, but will apply to the grant only.

Signal and Erealcd this third Day Of February 1976 [SEAL] Attest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner of Patents and Trademarks FORM PO-1OSO (10-69) uscoMM-Dc scam- 69 u.s, sovanumzur PRINTING orrucz; a 93 Q This is a contintfiathin of application Ser: No. 221.500 filed Jan. 38, 1972. now abandoned This invention relates generally to an improved process of photoelectrostatic duplication and more particular ly relates to a duplicating process in which the photocouductive member is reused in an environment where one or more high voltage electrodes and/or binsing electrodes are used in the reproduction process.

Photoclectrostatic duplicating processes, in which a photoconductive member is imparted a powder image which is then transferred to a sheet of plain paper, are generally well known. These duplicating processes require that the photoconductive layer be applied a blan ket sensitizing charge, followed by exposure to a pattern of light and shadow in order to create a charge image on the photoconductive surface. The charge image is visualized by development with an electroscopic powder. In many instances the developing device is equipped with a biasing electrode which applies a potential gradient between the conductive backing of the photoconductive member and the electroscopic powder. The need for such biasing electrodes depends on the particular system such as described, for example, in US. Pat. No. 3,598,580, granted on Aug. l0, 1970, to Evan S. Baltazzi, et al., entitled Photoelectrostatic Copying Process Employing Organic Photocon ductors and assigned to the same assignee, in which the photoconductive layer is underexposed and through the use of a biasing electrode at the developing station, the electrical field in the background area is rendered ineffective to attract toner.

The final step in the duplicating process requires the transfer of the powder image that has been developed at the developing station to a receiving sheet. Again, the transfer station utilizes electrodes to facilitate the movement of the powder particles from the photoconductive layer to the receiving sheet. Such electrodes may be corona discharge electrodes which impart a voltage to the receiving sheet having a polarity opposite to that of the toner particles causing the particles to move in the direction of the receiving sheet. A noncorona electrode may be employed which produces a field effect. such as a transfer roller having a potential applied thereto causing the powder to move in the direction of the receiving sheet.

It will be appreciated that the duplicating techniques referred to above, require the use of electrodes connected to high voltage sources. This requires that the photoconductive member be constructed in a manner capable of withstanding the deleterious effect of these high potential gradients. While the photoconductive member can be utilized in a variety of forms, it is convenient that the photoconductivc member referred to in this description be in the form of a continuous belt. it may also be in the form of a web in which case the phutoconductive member is carried on spools. being unwound from one spool and taken up on a wind-up spool. The spools are mounted in a rotatable drum in a manner hich permits a portion o'r the pliotownduo live member to be disposed on the drum surface. Rotating the drum cycles the pliotocontluctivc member pas the various image producing processing stations.

,453 Page 7 be deposited on the charge pattern: and during transfer, a complete circuit is required from the transfer electrode through ground to cause the powder to transfer to the receiving sheet.

Such a photoconductive member in the environment of an apparatus equipped with electrodes as described hcrcinubove, requires that the photoconductive film layer be uniform and continuous, free of any apertures or pin holes which would provide a direct connection between the conductive layer and the high voltage source. In the circumstance that there is a weakness, irregularity or failure in the photoconductive layer, there will result a short circuit between the electrode at the developing station, which will allow deposition of toner onto the photoconductive layer coextensive with the dimensions of the developer applicator device.

Such uncontrolled deposition of toner to the photoconductive sheet adjacent the areas where the short circuit occurs obliterates the image portions and hence, the member is no longer usable.

SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide a duplicating process for making reproductions on plain paper in which the construction of the photoconductive member permits a greater latitude in the range of operating parameters.

it is a specific object of the present invention to pro vide a duplicating process for making reproductions on plain paper in which consistently high quality copies can be obtained using high voltage biasing techniques without the risk of reducing the useful life of the photoconductive member.

It is a more specific object of the present invention to provide an improved copying process in which there are employed high voltage biasing electrodes at the various processing stations and which utilize a photoconductive member having a protective resistive layer so that the reproduction process is protected against the deleterious effects on the photoconductive member.

it is a further specific object of the present invention to provide a process which uses a photoconductive member constructed with a resistive layer in contact with the photoconductive layer to prevent the forma tion of short circuits through the photoconductivc layer at the processing stations which require high voltage biasing electrodes.

It is still another further specific object of this invention to provide an improved photocouductive member which is comprised of a conductive base support. a photoconductive layer and a resistive layer in contact with the photoconductive layer to prevent short circuiting currents to flow through irregularities it. the photoconductive layer.

Briefly, in the preferred embodiment of the r-'-. invention. the process of making a rcproducti: photocoutiucthe lilc'til' comp. isc ing a blanket eiectros. ic charge structcd photocondiactive medium Page ventional corona charging electrode; a charged photoconductive member is then exposed to a pattern of light and shadow resulting in the creation on the photoconductive member of a charge pattern. The charge pattern on the photoconductive member is developed by applying an electroscopic powder thereon utilizing a developing device, such as a magnetic brush, while imposing a potential gradient between the conductive basesupport of the electrophotographic member and the magnetic brush. The material image which is developed on the photoconductive layer is then transferred to a receiving sheet, such as plain paper under the influence of a transfer electrode, which applies a field between the surface of a powder image on the photoconductor and the plain paper which the image is to be transferred.

The success of the photoconductive member in withstanding exposure to the various high voltage electrodes as it courses past the processing stations, has been achieved by the novel and special construction of the photoconductive member thereby obviating the need for providing-a critically uniform, uninterrupted photoconductive layer. The special construction involves contacting the photoconductive layer with a protective resistive layer so as to provide an impedance between the developing brush and ground or the reference potential. The resistivity of the photoconductor in its dark adapted condition is about to 16 ohm centimeters; the resistivity of the protective resistive layer should be in the range of IO to 10 ohm centimeters.

The value of the resistivity for the current-limiting layer has been determined to be a function of the rate of movement of the photoconductor past the developing brush or electrode, and the biasing potentials applied. In general it can be stated that the layer must have a time constant which prevents loss of bias voltage at the site where there is a break in the photoconductive layer.

The generalrequirements of such a protective resistive layer is that it have an electrical resistance high enough to prevent the formation of shorting currents at the level where they do not result in loss of bias voltage at the photoconductive surface and thereby prevent the localized attraction of toner. The protective layer must also have a dielectric strength that is sufficiently high to prevent voltage breakdown. The resistivity should be low enough in order to produce negligible currentlimiting effects on the normal charging, developing and transferring operations under the influence of high voltage electrodes. It is to be understood that the protective layer must intervene between the electrode and the conductive layer and hence, it may be applied over the photoconductive layer or between the conductive layer and the photoconductive layer.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation of a duplicating apparatus capable of carrying out the process of the instant invention.

FIG. 2 is a schematic wiring diagram showing the various electrodes disposed about the photoconductive member.

FIG. 3 is a schematic in cross section showing one embodiment of a photoconductive member useful in the instant invention.

FIG. 4 is a schematic in cross section of another embodiment of the instant invention.

4 FIG. 5 is a schematic illustrating the improved per formance characteristics of the photoconductive members of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Having reference now to the drawings, and particularly FIG. 1, there is illustrated a copy machine 10 which is adapted to carry out the process of the instant invention. The copy machine 10 is of the photoelectrostatic type which is enclosed in a cabinet or housing 12, and is equipped with an infeed station 14, into which are fed the originals to be reproduced. At the lower portion of the housing there is provided a stack supply of cut sheets 16 from which supply are fed the copy sheets 17 or receiving members to which are transferred the powder images. The copy sheets 17 pass through a fixing station 18 and are delivered into a copy receiving tray 20 at the front of the housing 12.

In the forward portion of the housing is a photoconductive medium identified generally as 22, which is in the form of a continuous belt. The construction of the photoconductive medium 22 is critical to the successful operation of the instant process, as will be described in greater detail hereinafter. The photoconductive medium comprises a polyester base support 24 over which is applied a conductive layer 26. Directly in contact with the conductive layer 26 is the resistive protective layer 30 over which is next applied the photoconductive layer 32. It will be appreciated that the resistive layer 30 may be applied over the photoconductive layer 32. The continuous belt 22 is stretched about a plurality of rotatable rolls-34, 36, 38 and 40, which are mounted for rotation about horizontal parallel axes a, b, c and d, respectively. One of the rolls such as 40, is swingably mounted on two bracket members 42 and 43 and is designed for yieldably biased displacement away from the belt path to serve as a belt tensioning control. Another of these rolls, for example 36, is caused to be positively rotated through shaft b by a suitable driving mechanism comprising a motor 44 which drives the sprockets 41a and 41b through a drive belt 46, 'contained within the housing 12.

Surmounting the photoconductive medium 22 is a swingably mounted superstructure 47 formed of a pair of L-shaped members 48 and 49, only one such L shaped bracket 48 being visible in FIG. 1. The brackets are hingedly secured to the frame 50 through the hinged members 52 and held in spaced apart relation by the tie rods and bars 54 and 56, respectively. The superstructure 47 is swingably mounted into an operating position overlying the flat portion 58 of the path taken by the belt 22 passing over the rolls 34 and 38.

On the underside of the superstructure 47 is a corona discharge device 60 which includes a fine wire electrode 62 stretched inside a conductive shield 64. The corona discharge device extends between the members 48 and 49 being affixed to the tie bar 56. The corona discharge device is mounted in spaced relation above the photoconductive member 22, three-eights distance of from one-half to three-eighths of an inch, as the member passes over the roll 38. The photoconductive member 22 is imparted a blanket electrostatic charge at this station.

Included on the underside of the superstructure at the portion spanning the rolls 34 and 38, are a pair oi rotatable rolls 66 and 68 mounted for rotation about the shafts e and f, which. as shown in the drawings, are 

1. THE PROCESS OF MAKING REPREDUCTIONS FROM A REUSABLE PHOTOCONDUCTIVE MEDIUM COMPRISING THE STEPS OF: A. FORMING A PHOTOCONDUCTIVE MEDIUM CONSISTING ESSENTIALLY OF A CONDUCTIVE BASE SUPPORT CARRYING THEREON AN ORGANIC PHOTOCONDUCTIVE LAYER AND A RESISTIVE CURRENT LIMITING LAYER CONTAINING A FILM-FORMING MATERIAL SELECTED FROM THE GROUP CONSISTING OF METHYL CELLULOSE, POLYUREA, HYDROXYPROPYL METHYL CELLULOSE, ETHYLACRYLATE-ACRYLIC ACID-STYRENE TERPOLYMER, COPOLYMER OF METHYL VINYL ETHER AND MALEIC ANHYDRIDE AND POLYVINYL ALCOHOL, SAID CURRENTLIMITING LAYER HAVING A THICKNESS OF FROM ABOUT 2 TO ABOUT 50 MICRONS IN CONTACT WITH SAID PHOTOCONDUCTIVE LAYER, THE RESISTIVITY OF SAID RESISTIVE LAYER BEING IN THE RANGE OF 108 TO 1012 OHM-CENTIMETERS; B. APPLYING A BLANKET ELECTROSTATIC CHARGE TO THE PHOTOCONDUCTIVE MEDIUM BY MEANS OF A CHARGING ELECTRODE; C. EXPOSING THE CHARGED LAYER TO A PATTERN OF LIGHT AND SHADOW TO PRODUCE A CHARGE IMAGE THEREON; D. DEVELOPING THE CHARGE IMAGE BY APPLYING AN ELECTROSCOPIC POWDER THERINWHILE IMPOSING A POTENTIAL GRADIENT BETWEEN SAID CONDUCTIVE BASE AND THE ELECTROSCOPIC POWDER BY MEANS OF A BIASING POTENTIAL; E. TRANSFERRING THE POWDER IMAGE TO A RECEIVING MEMBER UNDER THE INFLUENCE OF A TRANSFER ELECTRODE; F. ESTABLISHING A REFERENCE POTENTIAL FOR THE ELECTRODES IN STEP (B), (D), AND (E), BY PROVIDING A REFERENCE ELECTRODE; G. REPEATING AT LEAST ONCE THE STEPS (B) THROUGH (F); WHEREBY LOSS IN THE BIASING POTENTIAL AT SAID DEVELOPING STEP IS PREVENTED BY SAID RESISTIVE LAYER INTERRUPTING THE DIRECT CONNECTION OF SAID PHOTOCONDUCTIVE LAYER TO SAID REFERENCE ELECTRODE IN THE EVENT OF ANY DISCONTINUITY IN SAID PHOTOCONDUCTIVE LAYER.
 2. THE PROCESS OF SUCCESSIVELY DEVELOPING AND TRANSFERRING A POWDER IMAGE FRM AN ELECTROPHOTOGRAPHIC MEMBER BEARING A LATENT ELECTROSTATIC IMAGE WITH AN ELECTROSCOPIC POWDER BY MEANS OF A MAGNETIC BRUSH DEVELOPER COMPRISING THE STEPS OF: A. FORMING A PHOTOCONDUCTIVE MEDIUM COMPRISING A CONDUCTIVE BASE SUPPORT CARRYING THEREON AN ORGANIC PHOTOCONDUCTIVE LAYER AND A RESISTIVE LAYER CONTAINING A FILM FORMING MATERIAL SELECTED FROM THE GROUP CONSISTING OF METHYL CELLULOSE, POLYUREA, HYDROXYPROPYL METHYL CELLULOSE, ETHYLACRYLATE-ACRYLIC ACID-STYRENE TERPOLYMER, COPOLYMER OF METHYL VINYL ETHER AND MALEIC ANHYDRIDE AND POLYVINYL ALCOHOL, SAID RESISTIVE LAYER HAVING A RESISTIVITY IN THE RANGE OF 108 TO 1012 OHM-CENTIMETERS AND HAVING A THICKNESS IN THE RANGE OF 2 TO 50 MICRONS IN CONTACT WITH SAID PHOTOCONDUCTIVE LAYER; B. CONNECTING SAID CONDUCTIVE BASE SUPPORT TO A REFERENCE ELECTRODE; C. APPLYING A BIASING POTENTIAL TO SAID MAGNETIC BRUSH THEREBY ESTABLISHING A PONTENTIAL GRADIENT BETWEEN SAID CONDUCTIVE BASE AND SAID ELECTROSCOPIC POWDER; WHEREBY ANY LOSS IN SAID BIASING POTENTIAL IS PREVENTED BY SAID RESISTIVE LAYER INTERRUPTING THE DIRECT CONNECTION OF SAID PHOTOCONDUCTIVE LAYER TO SAID REFERENCE ELECTRODE IN THE EVENT OF ANY DISCONTINUTIES IN SAID PHOTOCONDUCTIVE LAYER; D. AND THEREAFTER DEVELOPING THE CHARGE IMAGE BY APPLYING SAID ELECTROSCOPIC POWDER THEREON; E. AND TRANSFERRING THE POWDER IMAGE TO A RECEIVING MEMBER UNDER THE INFLUENCE OF A TRANSFER ELECTRODE; AND F. REPEATING AT LEAST ONCE THESTEPS (C) THROUGH (E).
 3. The method as claimed in claim 2 wherein the bias potential applied to the magnetic brush is in the range of 50 to 300 volts.
 4. The method as claimed in claim 2 wherein the reference electrode is at ground potential.
 5. The process of latent image duplication for making copies of a graphic original using a photoconductive medium comprising the steps of: a. forming a photoconductive medium comprising a conductive base support carrying thereon an organic photoconductive layer and a resistive current-limiting layer with a thickness of from about 2 to about 50 microns in contact with said photoconductive layer, the resistivity of said resistive layer being in the range of 108 to 1012 ohm-centimeters, said resistive current-limiting layer further characterized as containing a material selected from the group consisting of methyl cellulose, hydroxypropyl methyl cellulose, ethylacrylate-acrylic acid-styrene terpolymer, copolymer of methyl vinyl ether, maleic anhydride, polyvinyl alcohol and polyurea; b. applying a blanket electrostatic charge to the photoconductive medium by means of a charging electrode; c. exposing the charged layer to a pattern of light and shadow to produce a charge image thereon; d. developing the charge image by applying an electroscopic powder by means of a magnetic brush while imposing a potential gradient between said conductive base and the electroscopic powder by connecting said magnetic brush to a power source imposing a biasing potential at said developer; e. transferring the powder image to a receiving member under the influence of a transfer electrode connected to a voltage source establishing a potential gradient between the transfer member and said conductive support imposing a biasing potential during transfer; f. establishing a reference potential for the electrodes in steps (b), (d) and (e), by providing a reference electrode; g. repeating steps (d) and (e), making at least one additional transfer to said receiving member, whereby loss in biasing potentials at said developing and transfer stations is prevented by said resistive layer interrupting the direct connection of said photoconductive layer to said reference electrode iN the event of any discontinuities in said photoconductive layer.
 6. The method as claimed in claim 5 wherein the biasing potential at said developing station is in the range of 50 to 300 volts.
 7. The method as claimed in claim 5 wherein the biasing potential at the transfer station is in the range of 1,000 to 3, 000 volts.
 8. The method as claimed in claim 5 wherein the polarity of the applied voltage is the same as the polarity of said blanket electrostatic charge. 